*i have also bought a linksys router and it doesnt work*
Doesn't work at all with the Linksys? Did you power off the modem before connecting it to the Linksys? You MUST power cycle the modem whenever you connect it to a different device because the modem "locks" to the MAC address of the first device it sees connected to it (but I assume you know this since you've plugged your computer directly into it).
If the Linksys does the same thing as the Netgear then it has to be a modem issue. Even though it puzzles me why it would work okay directly connected to the computer.
You might also try configuring the router for a static IP. Once in a while I've run into a situation with a client's network where the router just doesn't reliably connect to the modem.
Go to the router's status menu page and write down the settings (IP, Gateway, DNS, Subnet Mask) and then go into the router's settings page and manually configure the router for a static IP on the WAN with those settings. Might be more stable.
Another possibility is that you're having DNS issues. Comcast sometimes has DNS problems. You can try setting up your computer for a static IP on the LAN and enter two alternate DNS server addresses listed below (these are Speakeasy Name Servers):
66.93.87.2
216.231.41.2
216.254.95.2
64.81.45.2
64.81.111.2
64.81.127.2
64.81.79.2
64.81.159.2
66.92.64.2
66.92.224.2
66.92.159.2
64.81.79.2
64.81.159.2
64.81.127.2
64.81.45.2
216.27.175.2
66.92.159.2
66.93.87.2